The present invention relates to a fuzzy controller for Anti-Skid Brake Systems (hereinafter referred as ABS), and particularly to a fuzzy controller which is capable of making various states of a vehicle fuzzy or indistinct so as to facilitate the fuzzy ABS control of a vehicle.
Most car drivers believe that they are capable of putting a full stop to a car in a short distance if they fully depress the brake-pedal. However, many experiments show that although fully depressing the brake-pedal will apply a large brake torque to car wheels, this will result in the locking of car wheels. Locking of car wheels will inevitably cause skidding and spinning of the car and accidents will thus happen.
When locked during braking, car wheels skid on the road and spinning occurs. If car wheels keep rolling with low slip ratio on the road during braking, static friction occurs between car wheels and the road. That is due to the fact that the point of the car wheel which contacts the ground will stay still relative to the ground at the instant of contact. In other words, no slip occurs between the car wheels and the road, if the car wheels are rolling (rather than skidding) during braking. As people skilled in this art know, dynamic friction coefficients are smaller than static friction coefficients. Thus keeping car wheels rolling during braking will result in larger frictional forces between the car wheels and the road. Furthermore, if car wheels keep rolling during braking, the car tends to move in the direction of the rolling wheels, and the driver is thus capable of steering the car in his/her own way during braking. If car wheels are locked during braking, car wheels will slip and move without regard to the direction of the wheels. Thus the car will spin out of control. Therefore, more and more cars are equipped with ABS which are provided with sensors for sensing tire speeds and inputting the speed data to a CPU installed within the system so as to assist the driver in preserving braking control.
There are presently many patents relating to ABS. These patents concern: structures for ABS, methods for controlling ABS, monitoring systems for ABS, . . . etc. Methods for controlling ABS can be divided into the following two categories:
The first category includes methods for monitoring the dynamic states (such as: angular speeds of car wheels, angular acceleration of car wheels, and slip differences between car wheels) of a car, methods for determining whether car wheels are locked or not, and methods for preventing the skidding of a car. The above-described methods are disclosed in U.S. Pat. Nos. 3,857,613, 4,363,523, 48,262,581, 4,835,695, 4,924,394, 5,063,514, 4,984,164, and 5,051,907. Among the above-described patents, the '613 patent disclosed an electric ABS, the '523 patent disclosed a structure for ABS, the '581 patent and the '695 patent disclosed a method for avoiding the skidding by monitoring the slip differences between each of the car wheels, the '394 patent suggested choosing the highest wheel speed as the speed of the car and calculating slip differences between each of car wheels and the speed of the car to prevent skidding, the '514 patent proposed use of slip differences as feedback signals and prevention of skidding by setting two threshold values for slip differences, the '165 patent proposed to use the tire speeds as feedback signals and by setting two critical values for tire speeds to prevent skidding, the '907 patent disclosed a method for preventing the locking of car wheels by monitoring the angular speeds of tires and controlling solenoid valves so as to decrease or increase the pressure exerted on brake linings.
The second category involves taking the friction coefficient between tires and the road into consideration and controlling the ABS precisely by sending appropriate commands into the system. The above-described methods are disclosed in U.S. Pat. Nos. 4,755,945, 4,836,616, 4,936,636, 4,991,103, and 5,058,018. Among the above-described patents, the '945 patent and the '616 patent proposed using a micro-processor for monitoring the variation of wheel speeds so as to determine whether the wheels are rolling on a high or low friction road and to control the ABS properly. The '636 patent, the '103 patent and the '818 patent disclosed methods similar to that of the '616 patent.
However, the above-described patents utilize a two value logic system in which only two values &lt;true&gt; and &lt;false&gt; are recognized. However, the real state of the car can not be distinctively divided into only two values. Thus, the above-described systems are not suitable for real-life applications.